1. Field of the Invention
The present invention relates to an antenna duplexer in which two filters of a high band side filter and a low band side filter are connected to an antenna port, and in particular relates to a phase shifter reducing interference between both filters.
2. Description of the Related Art
This kind of antenna duplexer is used as a duplexing means for transmitting and receiving waves in a high frequency wireless communication device such as a mobile communication terminal such as a cellular phone, a PHS, or a wireless LAN, or the like. A SAW (surface acoustic wave) filter is used for a high band side filter and a low band side filter. One of the high band side filter and the low band side filter is allocated to a transmitting side and the other is allocated to a receiving side, and both filters pass through only high frequency signals each having a specific frequency band and block signals of other frequency bands. When a plurality of SAW filters different in pass band are directly connected to the antenna in the antenna duplexer, respective transmitting and receiving signals interfere, so that provision of a phase shifter (phase matching circuit) is required.
FIG. 13 shows a conventional circuit structure of the antenna duplexer which includes a phase shifter. In the drawing, an antenna duplexer for an 800 MHz band US-CDMA is shown as an example. In general, these three of an antenna port, a high band side filter, and a low band side filter are required for an electric port. Each one system of “The high band side filter” and “the low band side filter” are installed inside the duplexer. In addition, the phase shifter is installed on the antenna port side of the high band side filter, and these are connected to point “A” just below the antenna port.
Specifically, a high band side filter 11 and a low band side filter 12 both include a plurality of SAW resonators 10 connecting in a ladder-shape, the high band side filter 11 takes the first stair on the antenna 13 side (the point A side) as a T-type input, and the low frequency filter 12 takes the first stage on the antenna 13 side as a π type input. A phase shifter 14 is inserted between the high band side filter and the antenna 13.
The phase shifter 14 is formed with an inductor and a capacitor in a π type. In other words, it forms a low band side filter configuration composed of an inductor 15 connected in series between the input/output terminal of the high band side filter 11 and the antenna 13, a capacitor 16 connected between the input/output terminal and the ground, and a capacitor 17 connected between the antenna 13 and the ground.
As these inductor 15, capacitors 16 and 17, a λ/4 microstrip line, lumped-constant circuits or the like are used (for instance, refer to Patent Document 1). In addition, a phase shifter in a high pass structure is known, in which a capacitor is disposed in series between an input/output signal of the high band side filter 11 and an antenna, and an inductor is disposed in parallel between both terminals of the capacitor and the ground (refer to Patent Document 2 for instance).
[Patent Document 1] Japanese Patent Application Laid-open No. 2004-228911
[Patent Document 2] Japanese Patent Application Laid-open No. 2006-129445